I built my current main/gaming system in late 2008, upgraded the video card from a Radeon HD4870 to a 5870 in October 2009, I upgraded the RAM from 4GB DDR2 to 8GB DDR3 in December 2010, and I installed a Blu-Ray RW drive in July of 2011.
Even though this system is 3 years old it is still more than sufficient for anything I use it for which includes hardcore gaming, video editing and encoding, photo editing, audio encoding, and writing and compiling programs. I think I can probably go at least another year or two before I need to upgrade the core components (CPU, motherboard, etc.), although I may upgrade when Ivy-Bridge comes out depending how they are performance wise.
I can definitely say I do major upgrades much less than I used to. Back in the Pentium 3 and 4 days I would often either completely rebuild or perform a major upgrade of core components every year. But now the power of computers has reached a point that core components can easily handle intense applications for 3 or 4 years only requiring upgrades of auxiliary components such as RAM, video cards, and drives to keep them relevant. This of course only applies if you don't cheap out and buy the minimum configuration that will just barely handle your needs at the time of the initial build/purchase.
